Motors (electric motors) that convert electrical energy into mechanical energy, which are used in various applications, typically include a rotor with a shaft that rotates about the shaft and a stator that is stationary relative to the rotor to magnetically interact with the rotor, and rotate the rotor by magnetic field (rotating magnetic field) that rotates and changes. Such motors are roughly classified, in view of the structure, into two motors, namely, a radial gap brushless motor (hereinafter abbreviated as “RG motor” where appropriate) and an axial gap brushless motor (hereinafter abbreviated as “AG motor” where appropriate). RG motors have a structure in which the stator and the rotor are spaced in the radial direction, and AG motors have a structure in which the stator and the rotor are spaced in the axial direction. AG motors, which have an advantage over RG motors in obtaining a larger torque with a small diameter, show promise, for example, for uses in vehicles.
Such AG motors are roughly classified into two types, namely, one with inner-rotor design (hereinafter abbreviated as “IR type” where appropriate) and one with outer-rotor design (hereinafter abbreviated as “OR type” where appropriate). The IR type AG motor has a structure in which coils are disposed at the stator and magnets are disposed at the rotor, and the rotor is disposed inside the stator. The OR type AG motor has a structure in which coils are disposed at the stator and magnets are disposed at the rotor, and the rotor is disposed outside the stator (for example, refer to Patent Literature 1). Characteristic differences between the IR type and the OR type lie in that the IR type has two times the number of coils that the OR type has whereas the OR type has two times the number of magnets that the IR type has. Although both of the IR type and OR type include a back yoke of a magnetic material outside the stator, the back yoke in the IR type functions as a return yoke in part of the magnetic circuit. Accordingly, the back yoke in the IR type is subjected to an AC magnetic field by the coils. To reduce resultant core loss, the back yoke in the IR type needs using a magnetic material that hardly allows eddy currents to flow therethrough, or for example, a laminated steel plate or compacted iron powder body. In contrast, the back yoke in the OR type, which is basically subjected to a DC magnetic field, only needs achieving magnetic shielding, and thus, pure iron based bulk iron material is good enough for the back yoke in the OR type. Accordingly the OR type AG motor, which includes a relatively small number of coils as assembly parts and does not need a relatively expensive magnetic material, are deemed advantageous in industrial applications compare with the IR type AG motor.
In addition, motors in general are required to be more compact and more powerful, or to have a high torque density. To achieve a higher torque density, driving a motor with a high magnetomotive force by passing a large current in the coils is required. In this motor driving at high magnetomotive force, the large current increases heat in the coils by copper loss or heat in the magnetic material by core loss. Accordingly to achieve a higher torque density, an efficient heat dissipation in the motor is required. The OR type AG motor allows for little space between the stator and the rotor to achieve an efficient magnetic effect between the stator and the rotor. Thus heat produced in the coils can hardly be released in the axial direction. Thus, the OR type AG motor has a structural disadvantage in view of heat dissipation.
A technique for heat dissipation in such an OR type AG motor is disclosed in, for example, Patent Literature 2. In the OR type AG motor disclosed in Patent Literature 2, a stator includes a housing that houses coils and their cores and a pump that feeds coolant by pressure into the housing to provide cooling for the coils and cores by means of the coolant that flows through the housing.
Meanwhile, although the OR type AG motor disclosed in Patent Literature 2 improves the heat dissipation by forced cooling by means of a coolant, the coolant come in contact with an outer peripheral surface alone of the coils. Coils in general are formed by winding a longitudinal conductor with insulation coating. Thus heat within the coil is conducted to the outer peripheral surface of the coil through insulation coating layers having poor thermoconductivity that are present between turns of the conductor. In this light, the OR type AG motor disclosed in Patent Literature 2 cannot be said as offering a preferable heat dissipation (cooling efficiency) for heat within the coil.
In addition, the OR type AG motor disclosed in Patent Literature 2 needs space for forming a flow path of the coolant between coils adjoining to each other in the circumferential direction, resulting in a decrease in the coil space factor in the circumferential direction. Thus, this motor would have a smaller number of coils among motors with the same size diameter, or would increase its size among motors with the same number of coils.